17-05-2012, 03:39 PM
Alternative Energy Sources for Automobiles
[attachment=22275]
An alternative to storing electrical energy in batteries is storing energy in the form of hydrogen. Hydrogen can be easily generated by electrolysis.
Electrolysis
1 gallon water produces 1250 gals (4.7 m3) of hydrogen (gas)
Power consumption: 4 kwh per 1m3 of H2 produced
Voltage required: 1.8 to 2.6 V
Liquid Hydrogen Properties:
H2 has very low density both as gas and liquid. Hence in spite of its high Calorific value on mass basis its energy density as a liquid is only 1/4th of gasoline.
H2 density = 0.07 g/cm3 ( gasoline 0.75, water 1.0)
Energy storage of H2 = 2.6 that of gasoline per unit mass
Liquid hydrogen needs 4 times the volume to give same energy as gasoline.
Storage
Hydrogen has to be stored as compressed gas, or as liquid (in cryogenic containers) or in absorbed form (as methyl hydrids), none of these is as convenient as gasoline storage.
Hydrogen Internal Combustion
Advantages
H2 has a high specific energy (energy per unit mass)
Higher than liquid hydrocarbons by a factor of 2.8
Very easy to burn
Main by-product is water vapor.
Can be operated as diesel engines at very lean air-fuel ratios and high compressions (15)
Spark ignited, but load variations are achieved using WOT (wide-open throttle) by varying the richness of the hydrogen-air mixture. (15)
The throttle valve can be omitted because H2 has broad flammability limits
Lower limit = 4%
Upper limit = 75%
Extremely lean mixtures during idle run leave a high concentration of unburned H2 (up to 3% by volume), which poses a safety problem.
Can be addressed by a compromise, using a throttle valve during idle run and WOT with variation in the mixture’s richness in all other cases. (15)
Disadvantages
Less horsepower
Shorter driving range
H-ICEs use 3.5 times more liquid H2 than gasoline
H2 has a very low energy density (energy per unit volume)
Liquid H2 has less energy density than aviation kerosene by a factor of about 4.1
Consequently provides less energy per storage space
Requires higher volume for the same amount of energy
Traces of NOx generated by high-temperature reactions of atmospheric N2 & O2 during combustion
Can be cleaned up by catalytic converters
Leaner air-fuel mixtures reduce combustion temperature but lower horsepower
Backfire is a major problem for H-ICEs
The air-fuel mixture can ignite before IVC (intake valve closure), causing an explosion in the intake manifold
Caused by hot spots in the combustion chamber
Residual gases
Surface deposits
Valves
Use of a lean air-fuel mixture eliminates backfire but decreases power output
The Promise of Fuel Cells
“A score of nonutility companies are well advanced toward developing a powerful chemical fuel cell, which could sit in some hidden closet of every home silently ticking off electric power.”
Theodore Levitt, “Marketing Myopia,” Harvard Business Review, 1960
Parts of a Fuel Cell
Anode
Negative post of the fuel cell.
Conducts the electrons that are freed from the hydrogen molecules so that they can be used in an external circuit.
Etched channels disperse hydrogen gas over the surface of catalyst.
Cathode
Positive post of the fuel cell
Etched channels distribute oxygen to the surface of the catalyst.
Conducts electrons back from the external circuit to the catalyst
Recombine with the hydrogen ions and oxygen to form water.
Electrolyte
Proton exchange membrane.
Specially treated material, only conducts positively charged ions.
Membrane blocks electrons.
Catalyst
Special material that facilitates reaction of oxygen and hydrogen
Usually platinum powder very thinly coated onto carbon paper or cloth.
Rough & porous maximizes surface area exposed to hydrogen or oxygen
The platinum-coated side of the catalyst faces the PEM.
Fuel Cell Operation
Pressurized hydrogen gas (H2) enters cell on anode side.
Gas is forced through catalyst by pressure.
When H2 molecule comes contacts platinum catalyst, it splits into two H+ ions and two electrons (e-).
Electrons are conducted through the anode
Make their way through the external circuit (doing useful work such as turning a motor) and return to the cathode side of the fuel cell.
On the cathode side, oxygen gas (O2) is forced through the catalyst
Forms two oxygen atoms, each with a strong negative charge.
Negative charge attracts the two H+ ions through the membrane,
Combine with an oxygen atom and two electrons from the external circuit to form a water molecule (H2O).
Hydrogen Fuel Cell Efficiency
40% efficiency converting methanol to hydrogen in reformer
80% of hydrogen energy content converted to electrical energy
80% efficiency for inverter/motor
Converts electrical to mechanical energy
Overall efficiency of 24-32%
Other Types of Fuel Cells
Alkaline fuel cell (AFC)
This is one of the oldest designs. It has been used in the U.S. space program since the 1960s. The AFC is very susceptible to contamination, so it requires pure hydrogen and oxygen. It is also very expensive, so this type of fuel cell is unlikely to be commercialized.
Advantages/Disadvantages of Fuel Cells
Advantages
Water is the only discharge (pure H2)
Disadvantages
CO2 discharged with methanol reform
Little more efficient than alternatives
Technology currently expensive
Many design issues still in progress
Hydrogen often created using “dirty” energy (e.g., coal)
Pure hydrogen is difficult to handle
Refilling stations, storage tanks, …
Electric Vehicle
The Electric vehicle is quiet, produces no exhaust emissions and is very efficient.
In an electric vehicle, the energy accumulator generally determines the vehicle performance.
The capacity of the motor is matched to the maximum output of the energy accumulator.
The energy accumulator is usually in the form of electrochemical battery or a fuel cell and its associated fuel tank.
[attachment=22275]
An alternative to storing electrical energy in batteries is storing energy in the form of hydrogen. Hydrogen can be easily generated by electrolysis.
Electrolysis
1 gallon water produces 1250 gals (4.7 m3) of hydrogen (gas)
Power consumption: 4 kwh per 1m3 of H2 produced
Voltage required: 1.8 to 2.6 V
Liquid Hydrogen Properties:
H2 has very low density both as gas and liquid. Hence in spite of its high Calorific value on mass basis its energy density as a liquid is only 1/4th of gasoline.
H2 density = 0.07 g/cm3 ( gasoline 0.75, water 1.0)
Energy storage of H2 = 2.6 that of gasoline per unit mass
Liquid hydrogen needs 4 times the volume to give same energy as gasoline.
Storage
Hydrogen has to be stored as compressed gas, or as liquid (in cryogenic containers) or in absorbed form (as methyl hydrids), none of these is as convenient as gasoline storage.
Hydrogen Internal Combustion
Advantages
H2 has a high specific energy (energy per unit mass)
Higher than liquid hydrocarbons by a factor of 2.8
Very easy to burn
Main by-product is water vapor.
Can be operated as diesel engines at very lean air-fuel ratios and high compressions (15)
Spark ignited, but load variations are achieved using WOT (wide-open throttle) by varying the richness of the hydrogen-air mixture. (15)
The throttle valve can be omitted because H2 has broad flammability limits
Lower limit = 4%
Upper limit = 75%
Extremely lean mixtures during idle run leave a high concentration of unburned H2 (up to 3% by volume), which poses a safety problem.
Can be addressed by a compromise, using a throttle valve during idle run and WOT with variation in the mixture’s richness in all other cases. (15)
Disadvantages
Less horsepower
Shorter driving range
H-ICEs use 3.5 times more liquid H2 than gasoline
H2 has a very low energy density (energy per unit volume)
Liquid H2 has less energy density than aviation kerosene by a factor of about 4.1
Consequently provides less energy per storage space
Requires higher volume for the same amount of energy
Traces of NOx generated by high-temperature reactions of atmospheric N2 & O2 during combustion
Can be cleaned up by catalytic converters
Leaner air-fuel mixtures reduce combustion temperature but lower horsepower
Backfire is a major problem for H-ICEs
The air-fuel mixture can ignite before IVC (intake valve closure), causing an explosion in the intake manifold
Caused by hot spots in the combustion chamber
Residual gases
Surface deposits
Valves
Use of a lean air-fuel mixture eliminates backfire but decreases power output
The Promise of Fuel Cells
“A score of nonutility companies are well advanced toward developing a powerful chemical fuel cell, which could sit in some hidden closet of every home silently ticking off electric power.”
Theodore Levitt, “Marketing Myopia,” Harvard Business Review, 1960
Parts of a Fuel Cell
Anode
Negative post of the fuel cell.
Conducts the electrons that are freed from the hydrogen molecules so that they can be used in an external circuit.
Etched channels disperse hydrogen gas over the surface of catalyst.
Cathode
Positive post of the fuel cell
Etched channels distribute oxygen to the surface of the catalyst.
Conducts electrons back from the external circuit to the catalyst
Recombine with the hydrogen ions and oxygen to form water.
Electrolyte
Proton exchange membrane.
Specially treated material, only conducts positively charged ions.
Membrane blocks electrons.
Catalyst
Special material that facilitates reaction of oxygen and hydrogen
Usually platinum powder very thinly coated onto carbon paper or cloth.
Rough & porous maximizes surface area exposed to hydrogen or oxygen
The platinum-coated side of the catalyst faces the PEM.
Fuel Cell Operation
Pressurized hydrogen gas (H2) enters cell on anode side.
Gas is forced through catalyst by pressure.
When H2 molecule comes contacts platinum catalyst, it splits into two H+ ions and two electrons (e-).
Electrons are conducted through the anode
Make their way through the external circuit (doing useful work such as turning a motor) and return to the cathode side of the fuel cell.
On the cathode side, oxygen gas (O2) is forced through the catalyst
Forms two oxygen atoms, each with a strong negative charge.
Negative charge attracts the two H+ ions through the membrane,
Combine with an oxygen atom and two electrons from the external circuit to form a water molecule (H2O).
Hydrogen Fuel Cell Efficiency
40% efficiency converting methanol to hydrogen in reformer
80% of hydrogen energy content converted to electrical energy
80% efficiency for inverter/motor
Converts electrical to mechanical energy
Overall efficiency of 24-32%
Other Types of Fuel Cells
Alkaline fuel cell (AFC)
This is one of the oldest designs. It has been used in the U.S. space program since the 1960s. The AFC is very susceptible to contamination, so it requires pure hydrogen and oxygen. It is also very expensive, so this type of fuel cell is unlikely to be commercialized.
Advantages/Disadvantages of Fuel Cells
Advantages
Water is the only discharge (pure H2)
Disadvantages
CO2 discharged with methanol reform
Little more efficient than alternatives
Technology currently expensive
Many design issues still in progress
Hydrogen often created using “dirty” energy (e.g., coal)
Pure hydrogen is difficult to handle
Refilling stations, storage tanks, …
Electric Vehicle
The Electric vehicle is quiet, produces no exhaust emissions and is very efficient.
In an electric vehicle, the energy accumulator generally determines the vehicle performance.
The capacity of the motor is matched to the maximum output of the energy accumulator.
The energy accumulator is usually in the form of electrochemical battery or a fuel cell and its associated fuel tank.